1. Field of the Invention
This invention relates to a flare stack for burning waste gas and more particularly to an igniter for maintaining combustion at the discharge end of the flare stack.
2. Description of the Prior Art
In many petroleum and chemical operations, combustible vapors are discharged from processing vessels or displaced from tanks as they are being filled. The vapors may be diluted with air or vary so widely in composition and rate of flow that they cannot effectively be processed. It is a conventional process to deliver such vapors into a flare stack and burn the vapors as they are discharged from the end of the flare stack. The flame at the discharge end of the stack may be extinguished by wind or rain or by a reduction in the flow rate or combustibles content of the gas below the level required to maintain combustion. It is desirable to provide means for igniting the flare. Because flares are frequently 60 or more feet high and because a large flame may be developed as the flares are ignited, it is desirable to provide automatic ignition means that are operated at a substantial distance from the discharge end of the flare stack.
U.S. Pat. No. 2,460,016 of Kuhn describes apparatus which includes a pilot flare that burns continuously near the discharge end of the stack. In the apparatus described, the pilot flare utilizes the same gas that is burned at the outlet of the flare stack to ignite the gases discharged from the stack. The pilot flare is exposed to substantially the same conditions which make maintenance of combustion at the discharge end of the stack difficult. U.S. Pat. No. 2,460,016 describes means for igniting the pilot flare by initiating combustion near the bottom of an ignition tube. A flame travels upwardly through the ignition tube to ignite combustible gas delivered to the pilot flare.
U.S. Pat. No. 2,537,091 of Rodman et al describes apparatus generally similar to the apparatus described in U.S. Pat. No. 2,460,016, with the exception that a separate supply of fuel gas, rather than the waste gas, is used for the pilot flare. A manually operated spark plug initiates a flame which travels to the top of the stack through a riser. A stream of the fuel gas is discharged directly into the stack and is ignited at the discharge end thereof by the flame from the riser to provide the pilot flare.
U.S. Pat. No. 3,833,336 of Ray describes apparatus in which a fuel such as LPG is utilized to maintain a pilot flare near the top of the flare stack. The fuel is also discharged, in response to a signal indicating the pilot flare is out, into the lower portion of an open bottomed igniter pipe. The fuel discharged into the lower end of the pipe aspirates air into the pipe and the resultant mixture is ignited by a spark plug adjacent the outlet of the fuel supply line. If the pilot flare should go out, a signal actuated manually or by a circuit in response to a flame detector opens a valve in the fuel gas line to the igniter pipe and activates the spark plug.
To avoid the necessity of the continued observance of the upper end of the stack or a flame detector, an ignition system has been developed in which a fuel gas supplied to an igniter aspirates combustion air into the fuel gas stream and is ignited intermittently, for example, every 5 seconds by a spark plug. The resulting flame travels to the discharge end of the flare stack through a conduit in which there is a combustible mixture of a fuel gas and air and ignites waste gas at the discharge end of the flare stack. The repeated ignition avoids the possibility of the flare being extinguished for a period adequate to allow any substantial discharge of unburned waste gases from the top of the stack. The intermittent operation resulting from the periodic spark causes surges in pressure with resultant flashback and noise.